Beirut: Islamic State has beheaded or shot 17 men in Syria in the past two days as it faces increasing attacks from unidentified gunmen in areas it controls, a monitoring group said on Friday.

The Syrian Observatory for Human Rights, which uses a network of sources across the country to report on the war, said the spike in execution-style killings follows the killing of 10 Islamic State fighters in hit-and-run attacks in several areas around Deir al-Zor province this month.

Deir al-Zor stretches from Raqqa to the border with Iraq and links Islamic State`s self-declared caliphate in the two countries.

Syrian state news agency SANA said on Thursday Islamic State executed three civilians and displayed their bodies in the town of al-Mayadin.

Islamic State fights in Syria against both forces loyal to President Bashar al-Assad and rival insurgents in a war that is in its fourth year and which the United Nations says has killed 200,000 people.

There have been no claims of responsibility for the recent attacks, attributed by Syrian state media last week to "popular resistance" to Islamic State.

Last year, a news agency reported on Syrians who say they have been hunting down Islamic State fighters in Deir al-Zor province operating in small groups in a guerrilla campaign that has emerged as a response to IS brutality.